1. Field of the Invention
The present invention relates to a method of producing a solar cell; a solar cell, and a method of producing a semiconductor device. In particular, it relates to a method of producing a silicon solar cell which realizes electrical separation of the p n junction necessary for forming the electrodes easily and with a high productivity; a solar cell having the p n junction which is electrically separated in a simple manner, and a method of producing a semiconductor device in which the electrode capable of reducing substantially the contact resistance to the semiconductor substrate by applying the same physical phenomenon as in the electrical separation of the p n junction in forming the electrodes of the solar cell.
2. Discussion of Background
Now, silicon solar cells form the main stream for power solar cells on the earth. In general, process flows in large scale production have to be simple as possible to reduce manufacturing cost.
In the following, description will be made as to an example of a manufacturing process with use of FIG. 17 which shows a diagram of manufacturing steps of a solar cell.
FIG. 17a shows a p type Si substrate 1. In FIG. 17b, phosphorus (P), for instance, is thermally diffused into the p type Si substrate to form an n type diffusion layer 2 by inverting an electric conduction type.
In general, phosphorus oxychloride (POCl.sub.3) is often used as a diffusion source of phosphorus. Further, when there is no special contrivance, the n type diffusion layer is formed in the entire surface of the p type Si substrate 1. The sheet resistance of the n type diffusion layer is about several tens .OMEGA./.quadrature. and the depth of the diffusion layer is about 0.3-0.5 .mu.m.
The n type diffusion layer 2 is applied with a resist on surfaces except for the principal plane (light incident plane), and an etching process is effected so that the n type diffusion layer 2 remains only in the principal plane (FIG. 17c). The resist is removed by using an organic solvent.
Then, screen printing with an aluminum paste 3 is conducted to the surface opposite to that in which the n type diffusion layer is formed, and the substrate is baked in a near-infrared furnace at 700.degree. C.-900.degree. C. for several min to ten and several min. As a result, aluminum as an impurity is diffused from the aluminum paste into the p-type Si substrate 1 whereby a Back Surface Field (BSF) layer 4 as a p.sup.+ layer including an impurity at a high concentration is formed as shown in FIG. 17d. The BSF layer 4 contributes the energy conversion efficiency of the solar cell.
Then, a comb-shaped pattern of silver paste is screen-printed to form an n electrode 5 on the n type diffusion layer 2 which is formed only in the principal plane as shown in FIG. 17e. The substrate with the n type diffusion layer 2 and the n electrode 5 is baked at 700.degree. C.-800.degree. C. whereby a solar cell is completed.
Although illustration is omitted for simplifying description, there are many cases that an antireflection film such as TiO.sub.2, SiN, SiO.sub.2 or the like is formed on the n type diffusion layer 2 in order to improve the energy conversion efficiency.
FIG. 17f shows a structure of a solar cell produced according to the process similar to the above-mentioned process wherein the n type diffusion layer 2 is remained in side surfaces of the p type Si substrate 1 in addition to the principal plane. In this case, a portion indicated by reference numeral 6 has a P.sup.+ /n junction. The concentration of an impurity (phosphorus) in or near the surface of the n type diffusion layer 2 is very high as the order of 10.sup.20 -10.sup.21 cm.sup.-3, such a structure being apt to cause electrical leaking. As a result, the energy conversion efficiency of the solar cell is remarkably reduced. Namely, if the n type diffusion layer is not formed only in a single plane (light incident plane) as shown in FIG. 17c, a solar cell having excellent characteristics can not be manufactured. In the conventional technique, as described above, the application of a resist to protect the surface of the diffusion layer, the etching treatment and the removal of the resist are required. In order to reduce the manufacturing cost, however, it is necessary to simplify these processes as possible.